Two features of the new materials which may result from the modification of these clays or of aluminium-bearing minerals are an enhanced capacity to exchange cations from solution (i.e. a cation exchange capacity) and/or an increase in the available surface area when compared with the properties of the initial starting mineral (e.g. clay or zeolite). These two features are of considerable significance to the cost-effective use of these derivative materials in a wide range of applications for cation-exchange (e.g. for removal of toxic metal ions from aqueous and non-aqueous solutions; removal of NH.sub.4.sup.+ from aqueous and non-aqueous solutions, as detergent builders and as water softeners), absorption (e.g. for the removal of gases from the environment, for absorption of cations from solutions), as agents for the controlled release of desired cations into an environment and as substrates for catalysis reactions in the modification of hydrocarbons and other chemicals.
Clay minerals are part of the larger family of minerals called phyllosilicates--or "layer" silicates. These clay minerals are typically characterised by two-dimensional arrangements of tetrahedral and octahedral sheets, each with specific elemental compositions and crystallographic relationships which define the mineral group. Thus,the tetrahedral sheet may have the composition T.sub.2 O.sub.5 (where T, the tetrahedral cation, is Si, Al and/or Fe) and the octahedral sheet may commonly contain cations such as Mg, Al and Fe, but may also contain other elements such as Li, Ti, V, Cr, Mn, Co, Ni, Cu and Zn (Brindley and Brown, 1980, Crystal structures of clay minerals and their X-ray identification, Mineralogy Soc., London). Each of these clay mineral groups can be further classified into trioctahedral and dioctahedral varieties, depending on the occupancy of the octahedra in the respective sheet arrangement(s). Some specific mineral species may show cation occupancies which are intermediate between the two varieties. Nevertheless, the relative arrangement of these tetrahedral and octahedral sheets also defines the basic mineral groups in that an assemblage which links one tetrahedral sheet with an octahedral sheet is known as a 1:1 layer type mineral. An assemblage which links two tetrahedral sheets with one octahedral sheet is known as a 2:1 layer mineral. This basic classification of mineral species, based upon the crystallographic relationships of specific sub-units, is well-known by those skilled in the art of clay mineralogy and forms a basis for description of this invention.
Notwithstanding the crystallography of these sub-units within clay minerals, the alumino-silicate derivatives of this invention also include minerals which contain a tetrahedral framework of oxygen atoms surrounding either silicon or aluminium in an extended three-dimensional network. For example, various zeolites contain different combinations of linked tetrahedral rings, double rings or polyhedral units, but they are also amenable to provide an alumino-silicate derivative (hereinafter referred to as "ASD") of the invention.
The production of an amorphous derivative, termed "kaolin amorphous derivative" (KAD) from kaolin clays which are 1:1 alumino-silicates, has been described in an earlier disclosure (WO95/00441). This specification describes the production of KADs from the kaolin clay staring material by reaction of the kaolin clay with an alkali metal halide MX where M is alkali metal and X is halide.
In this specification, the reference to MX was the only example of a suitable reagent which could convert the majority of the octahedrally coordinated aluminium in the kaolin group mineral to tetrahedrally coordinated aluminium. However, no reference was made to any possible mechanism by which this phenomenon occurred.
However, surprisingly it has now been discovered that an alternative reagent such as a highly basic solution in the form of MOH where M is an alkali metal cation can provide a similar result wherein the majority of the octahedrally co-ordinated aluminium can be converted to tetrahedrally co-ordinated aluminium.
Without wishing to be bound by theory, it is hypothesised that a reagent which can achieve this particular result may comprise a compound that disassociates into cationic species and anionic species such that hydroxyl ions are present in a concentration which is in excess compared to the concentration of hydrogen ions. In addition to this feature or in the alternative, the compound causes to be formed in the resulting solution due to interaction with the alumino-silicate mineral, hydroxyl ions in excess concentrations compared with the concentration of hydrogen ions.
With the formation of excess hydroxyl ions, it would seem that such excess hydroxyl ions result in reconstruction of cation-oxygen bonding within the starting material such that a stable, amorphous material with the abovementioned desirable properties may be formed.
Again, while not wishing to be bound by theory, this chemical transformation or conversion may be represented by the following example in which kaolinite, with Al and Si in octahedral and tetrahedral sites in the kaolinite structure, respectively, is reacted with an alkali metal halide where the cation is K.sup.+ or an ammonium ion in an aqueous solution such that excess halide (e.g. X.sup.-) is readily exchangeable with the available hydroxyl groups (OH.sup.-) in the kaolinite structure. This exchange results in the formation of a highly basic solution with an excess of OH.sup.- ions which can cause rearrangement of octahedrally co-ordinated aluminium through the action of these OH.sup.- ions on hydrogen-bonded oxygen atoms. This rearrangement of aluminium co-ordination results in primarily tetrahedrally co-ordinated aluminium in this resultant stable material. This therefore provides a suitable explanation why MX was a suitable reagent in the case of W095/00441.
Alternatively, a highly basic solution can be generated by the use of a reagent such as a compound which disassociates into cationic and anionic species. The anions, present in excess, may also cause the rearrangement of octahedrally co-ordinated aluminium to tetrahedrally co-ordinated aluminium through their action on hydrogen-bonded oxygen atoms. Other examples of this type of chemical transformation of clays include the reaction of kaolinite or montmorillonite with a caustic reagent (e.g. MOH; where M is a cation such as K.sup.+, or Na.sup.+ or Li.sup.+) such that rearrangement of octahedrally co-ordinated aluminium to tetrahedrally coordinated aluminium through their action on hydrogen-bonded oxygen atoms occurs.